Multiple sclerosis (MS) is a chronic inflammatory disorder of the central nervous system (CNS) and one of the most common causes of nontraumatic disability among young and middle-aged people. One of the hallmarks, however, is the progressive neurodegeneration that plays a key role in the progression of neurological disabilities. Little is known of the link between neuroinflammation and neurodegeneration. Recent biochemical studies suggested that there is defective oxygen metabolism in mitochondria due to increased nitric oxide (NO) as a result of vascular inflammation, which may play a crucial role in neuronal/axonal injury. In addition, NO is a strong mediator of neurovascular coupling that is responsible for increased blood supply during transient neural activation. In MS, the presence of a tonically high NO level (even during resting) may desensitize the vascular smooth muscle over time with a consequence of decreased vasodilatory capacity or cerebral vascular reactivity (CVR) and limited blood supply when neurons perform a demanding task. The Overarching Goals of this proposal are to detect and characterize abnormalities in oxygen consumption and vascular reactivity in early MS and identify tissues at risk using several advanced metabolic/vascular MRI techniques. These include a recently developed T2-Relaxation-Under-Spin-Tagging (TRUST) for the evaluation of global cerebral metabolic rate of oxygen (CMRO2) and a patient-comfortable blood-oxygen-level-dependent (BOLD) paradigm using CO2 inhalation to measure CVR. We will quantify CMRO2 and CVR abnormalities in patients with early relapsing-remitting (RR) MS and subsequent advanced stage of secondary progressive (SP) MS that are associated with clinical disability and disease progression. We hypothesize that oxygen metabolism abnormality in conjunction with impaired blood flow regulation is a key factor causing early degeneration. We also hypothesize that the combined functional index of CMRO2 and CVR has the potential to be an objective marker to predict neurodegenerative progression and its clinical outcome in MS. The Specific Aims are as follows: 1. To assess global CMRO2 abnormalities using TRUST MRI in patients with early RR and SP patients as compared with age/sex matched normal controls; 2. To measure CVR using inhalation of 5% CO2 and BOLD imaging in order to elucidate the nature of cerebrovascular dysfunction in MS patients; 3. To measure CVR and CMRO2 changes in healthy controls between groups with low and high intake of dietary nitrate (corresponding to higher NO level); 4. To determine longitudinal changes of CMRO2 and CVR in early RRMS patients and their relationship to imaging and clinical outcomes over a 5-year period. Health Relevance: By providing in vivo MRI confirmation of our working hypotheses, this application could have profound consequences for our understanding of disease pathogenesis and progression (neurological disability and cognitive decline) in MS, and for the future design of novel therapeutic strategies.